This invention relates to a method for predepositing dopant material on semiconductor substrates, and especially to a method for predepositing impurity dopant on a semiconductor substrate from a high pressure plasma source.
In the fabrication of most semiconductor devices localized doped regions are formed in a semiconductor substrate to form p-n junctions or to form regions of high dopant concentration within a background of lower dopant concentration. The doped region is usually formed by predepositing a layer of doped oxide or doped glass on the surface of the substrate and then subsequently heating the substrate to an elevated temperature. At the elevated temperature the dopant in the doped layer is redistributed by thermal diffusion of the dopant into the semiconductor substrate as well as within the layer. It has been conventional to predeposit the doped layer of glass or oxide on the substrate by reacting a dopant material with an oxide former, typically in a diffusion furnace at an elevated temperature. Representative reactions include the reaction of diborane (B.sub.2 H.sub.6) with oxygen to form B.sub.2 O.sub.3, or the reaction of phosphine (PH.sub.3) with oxygen to form P.sub.2 O.sub.5.
While predeposition of a doping source from a thermal reaction as described above has long served as an acceptable predeposition method, with the advent of larger and larger substrates and with the need for high throughputs in diffusion processes, the inadequacies of these prior art methods are becoming apparent. In order to achieve a high throughput in a predeposition process, a large number of these semiconductor substrates must be accommodated in a high temperature reaction apparatus in each process run. The high number of wafers in the apparatus can be achieved by closely spacing the substrates, but closely spaced, large sized substrates lead to nonuniformities in the deposition of the dopant glass. The nonuniformities result from the dynamics of the gas flow as the reactant gases including the dopant material pass down the predeposition reactor and diffuse into the narrow spaces separating the substrates. The gas flow dynamics are such that a relatively thicker deposit of dopant material is formed near the periphery of the substrates and a relatively thinner, less highly doped deposit is provided in the center of the substrates. Another factor which contributes to the nonuniformity of the resultant diffusion is that the predeposition is done at an elevated temperature. At the elevated temperature significant diffusion is occurring during the predeposition. Because of this, the junction depth at the substrate periphery is deeper than near the substrate center.
In addition to problems with nonuniformity, predeposition of a doped oxide layer from which the dopant material is diffused results in the formation of an insulating layer on the surface of the substrate which subsequently must be removed at least locally to allow electrical contact to the doped region. Removing this insulating layer requires an additional processing step.
The disadvantage of the nonuniform deposit can be overcome by placing fewer substrates in the reactor, by increasing the spacing between the substrates, or by placing the substrates flat within the reactor rather than "coin stacking" them in a stand up, face-to-face relationship. These solutions, however, result in a lowered throughput which, of course, increases the cost of the predeposition process.
The problem with nonuniform predeposition of dopant material as well as the problem of depositing an insulating material on the substrate surface, which must be subsequently removed, can both be overcome by an ion implantation predeposition process. In this process, the dopant material is implanted directly into the surface of the substrate. No insulating layer need be formed on the substrate surface and the ion implantation can be relatively uniform and controllable across the surface of a substrate and from substrate to substrate. Ion implantation equipment, however, is not amenable to high volume, high throughput processing. Additionally, ion implantation equipment is expensive. The ion implant process must be carried out at low pressures which requires vacuum equipment with all of the problems associated therewith, as well as long cycle times associated with vacuum pump down and the like.
Accordingly, there is presently a need for an improved predeposition method which will overcome the disadvantages of these prior art methods to provide uniform diffusion predepositions at a high throughput for the processing of semiconductor devices and especially large area semiconductor devices such as photovoltaic devices.
It is therefore an object of this invention to provide a high pressure plasma process for predepositing dopant upon a semiconductor substrate.
It is a further object of this invention to provide an improved process for preferentially depositing dopant on a selected surface of a semiconductor substrate.
It is another object of this invention to provide an improved method for predepositing dopant upon a semiconductor substrate without also forming an insulator film on the substrate surface.
It is yet another object of this invention to provide an improved method for predepositing different dopant type impurities upon the different major surfaces of a semiconductor substrate.